Along with the recent explosive growth of the Internet, LANs (Local Area Networks) have been set up in more offices and households. Due in part to improvements in digital wireless communications technologies, there has been a rapidly growing need for a wireless LAN (i.e., a LAN that is set up wirelessly in order to remove the burden of cabling). Furthermore, partly because the wireless LAN can be used in a mobile environment of a mobile terminal as typified by a laptop computer, the wireless LAN is expected to be used in appreciable numbers in the future. A representative technology of the wireless LAN is the IEEE 802.11, which is standardized by the IEEE (Institute of Electrical and Electronics Engineers). This standardized technology defines an OSI (Open System Interconnection) model from a physical layer to an MAC (Media Access Control) layer, which serves as a lower layer of a data link. Further, the IEEE 802.11 can replace the Ethernet (registered trademark), which is a transmission channel in the wired LAN. Furthermore, the IEEE 802.11 is specified to provide a roaming function, which is an optional function peculiar to a wireless method.
Further, new terrestrial digital broadcasting is now being prepared to replace terrestrial analog broadcasting, which is currently viewed in Japan. Terrestrial digital broadcasting was initiated in 2003 in three major metropolitan areas of Kanto, Kinki, and Tokai, and will be expanded nationwide by 2006. Accordingly, the existing analog broadcasting will be discontinued by 2011.
ISDB (Integrated Services Digital Broadcasting) is a concept of next-generation integrated digital broadcasting which handles, as digital data, various types of information such as video, audio, and data. Digital television broadcasting, digital audio broadcasting, facsimile broadcasting, multimedia broadcasting, and other types of digital broadcasting are being studied as specific ISDB services. Satellite broadcast waves, terrestrial broadcast waves, and cable transmission paths such as coaxial cable and fiber optics may be used as ISDB transmission paths.
In a terrestrial digital broadcasting technical standard called the ISDB-T (Terrestrial) standard, OFDM (Orthogonal Frequency Division Multiplexing), which uses a large number of carriers, is adopted as a modulation method. This makes it possible to suppress ghost interference caused by a plurality of propagation paths (multi-path) formed when, e.g., each of the carriers is reflected by a building. Further, the ISDB-T standard specifies a plurality of transmission modes which define carrier intervals, a plurality of carrier-specific modulation methods, and a plurality of time-axial guard intervals which are set up every effective symbol length, thereby theoretically allowing for an extremely large number of signal formats. Practically, an optimum format is selected among the signal formats, in accordance with a service such as fixed reception or mobile reception.
Further, in the ISDB-T standard, a single transmission channel (communication channel) (having a bandwidth of approximately 5.6 MHz) is divided into thirteen segments (each of which occupies approximately 430 kHz), and the modulation methods are changed in accordance with the thirteen segments as a unit. This enables a broadcasting station to arbitrarily decide a signal arrangement by, e.g., allocating a signal transmission channel to audio broadcasting and Hi-Vision broadcasting, or by allocating a single transmission channel to standard fixed broadcasting and mobile broadcasting.
Furthermore, the ISDB-T incorporates time-axial interleaving and uses a radio wave suitable for mobile transmission. Therefore, one important characteristic of the ISDB-T lies in that stable reception is made possible at a mobile receiver (e.g., an on-board television) and a portable terminal (e.g., a PDA (Personal Digital Assistant) or a mobile phone). In future, there is a high degree of expectation for a service assuming such mobile reception.
Incidentally, a bandwidth of data transmission is reduced when a pair of a transmitter and a receiver communicates with each other by using a specific frequency (a transmission channel) at which another pair of a transmitter and a receiver communicates with each other. In order to avoid this, the devices about to use the transmission channel already occupied by the other devices need to be automatically transferred onto an unused transmission channel.
For example, Patent Document 1 discloses a channel-switching wireless communication device including a wireless communication section. The wireless communication section is provided with a 2.4 GHz band front-end circuit and a 5 GHz band front-end circuit so as to accommodate to the 2.4 GHz and 5 GHz bands. This makes it possible, in a wireless LAN system, to greatly increase the number of transmission channels which can be set simultaneously in the same area. This allows reduction of the risk of an interfering radio wave interrupting a communications link.
[Patent Document 1]    Japanese Unexamined Patent Publication No. 33676/2002 (Tokukai 2002-33676; published on Jan. 31, 2002) (FIG. 1)
However, the transmission channel switching in such a conventional wireless communication device causes interruption of the communication, so that an image is not displayed. Therefore, the conventional wireless communication device has such a problem that a user has no way of knowing whether an image has been interrupted or a communication terminal has switched transmission channels.
The present invention is made in view of the foregoing problems and has as an object to provide a wireless terminal, a base device, a wireless system, a wireless terminal control method, a wireless terminal control program, and a computer-readable storage medium storing the wireless terminal control program, all of which prevents the user from being bothered by a feeling of discomfort even when image display is interrupted due to the switching of the transmission channels or the like, thereby improving user-friendliness.
Further, the present invention has an object to provide a wireless terminal, a base device, a wireless system, a wireless terminal control method, a wireless terminal control program, and a computer-readable storage medium storing the wireless terminal control program, all of which can maintain a communication condition as optimum as possible in a network as a whole.